An active pixel sensor ("APS") is a light sensing device with sensing circuitry inside each pixel. Each active pixel includes a light sensing element and one or more active transistors within the pixel itself. The active transistors amplify and buffer the signals generated by the light sensing elements in the pixels. In comparison with the widely used charge coupled devices (CCDs), an APS device has a number of unique and beneficial features. These features include the ability to receive and process input signals with the active pixels without the charge transfer process inherent in CCDs. An APS device is also compatible with CMOS processes.
Elimination of the charge transfer allows APS devices to have a higher readout rate than those of CCDs and also to maintain their performance as the array size increases. Compatibility with CMOS processes allows many signal processing functions and operation controls to be integrated on an APS chip. Use of CMOS circuitry with APS devices also reduces cost of manufacturing and power consumption. Moreover, the active pixels of APS devices allow non-destructive readout and random access.
One configuration of APS sensors is disclosed in U.S. Pat. No. 5,471,515 by Fossum et al., the disclosure of which is incorporated herein by reference. Such an image sensor typically operates at a constant finite frame rate with a constant signal integration time. The frame rate and the integration time are often preset for normal operating conditions under which illumination is usually sufficient. The performance of such a conventional image sensor suffers when the illumination is below the typical level to which the image sensor is configured. Since the frame integration time is usually preset for a predetermined normal light condition, little signal may be collected under a low light condition with an illumination level below the predetermined normal light condition. Conversely, too much signal may also be collected when input light level is higher than the predetermined level. Thus, a clear image of a target can be difficult to obtain under different light conditions.
One conventional technique to circumvent this problem under low light conditions uses averaging the signals from a plurality of neighboring pixels in order to reduce the noise level. This averaging technique improves the signal-to-noise ratio of the image sensor. The averaging process, however, also reduces the image resolution. Suppose a patch of N.times.N pixels are averaged, the signal-to-noise ratio is then improved by a factor of N.sup.1/2 while the resolution is reduced by a factor of N. One limitation of this averaging technique is that the signal level achieved from averaging essentially remains unchanged compared to the signal levels of the selected neighboring pixels. Additional signal amplification is thus needed to obtain a brighter image.